Title Comparison of lower limb and back exercises for runners with chronic low

back pain Author(s) Congcong Cai, Yifan Yang and Pui W. Kong Source Medicine & Science in Sports & Exercise, 49(12), 2374-2384 Published by Human Factors and Ergonomics Society Copyright © 2017 Human Factors and Ergonomics Society This is the author’s accepted manuscript (post-print) of a work that was accepted for publication in the following source: Cai, C., Yang, Y., & Kong, P. W. (2017). Comparison of lower limb and back exercises for runners with chronic low back pain. Medicine & Science in Sports & Exercise, 49(12), 2374-2384. https://doi.org/10.1249/MSS.0000000000001396 Notice: Changes introduced as a result of publishing processes such as copy-editing and formatting may not be reflected in this document.

1

Comparison of Lower Limb and Back Exercises for Runners with Chronic Low Back Pain

Congcong Cai, PT, Ph.D.1,2, Yifan Yang, Ph.D.2 and Pui W. Kong, Ph.D.2

1. Physical Education and Sports Science Academic Group, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616.

2. Physiotherapy, Rehabilitation Department, Ng Teng Fong General Hospital-Jurong Health Services, 1 Jurong East Street 21, Singapore 609606.

Corresponding Author: Dr Pui Wah Kong Physical Education and Sports Science Academic Group, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616.

Tel: +65 6219 6213

Email: puiwah.kong@nie.edu.sg

2

Comparison of Lower Limb and Back Exercises for Runners with Chronic Low Back 1

Pain 2

ABSTRACT 3

Introduction: This single-blind randomized trial was conducted to compare the treatment 4

effect of lower limb (LL) exercises versus conventional lumbar extensor (LE) and lumbar 5

stabilization (LS) exercises in recreational runners with chronic low back pain (cLBP), 6

since there is currently no specific protocol for managing runners with cLBP. 7

Methods: 84 recreational runners with cLBP were allocated to three exercise groups (LL, 8

LE, LS) for an 8-week intervention. Outcome measures included self-rated pain and 9

running capability, lower limb strength, back muscles function, and running gait. 10

Participants were assessed at pre-, mid- and end-intervention; selected outcomes also 11

followed up at three and six months. Generalized estimating equation was adopted to 12

examine group-by-time interaction. 13

Results: LL group improved 0.949 points per time point in Patient Specific Functional 14

Scale (p < .001), which was higher than the LE (B = -0.198, p = .001) and LS groups (B = -15

0.263, p < .001). All three groups improved on average 0.746 points per time point in 16

Numeric Pain Rating Scale for running induced pain (p < .001). Knee extension strength 17

increased 0.260 Nm/kg per time point (p < .001) in the LL group, which was higher than 18

the LE (B = -0.220 Nm/kg, p < .001) and LS groups (B = -0.206, p < .001). LL group also 19

showed greater increase in running step length (2.464 cm per time point, p = .001) than LS 20

group (B = -2.213, p = .013). All three groups improved similarly in back muscles function. 21

Conclusion: LL exercise therapy could be a new option for cLPB management given its 22

superior effects in improving running capability, knee extension strength, and running gait. 23

Key Words: legs, lumber extensor, lumbar stabilization, spine.24

3

INTRODUCTION 25

The prevalence of chronic low Back Pain (cLBP) among recreational runners was reported 26

as high as 13.6% in the United States (56). Low back injuries accounted for about 7% of all 27

running injuries (15) but there is no runner-specific exercise therapy protocol in the current 28

management of cLBP. The clinical management of runners with cLBP is largely based on 29

protocols for the general population which were built upon the understanding of fatigability 30

(28, 47) and strength deficit (16, 23) of the lumbar extensor muscles, and/or compromise in 31

motor control of the lumbar stabilizing muscles (12, 22). Unfortunately, randomized trials 32

investigating the treatment effect of lumbar extensor and lumbar stabilization exercise 33

therapies have shown inconclusive results in managing cLBP in the general population (20, 34

26, 35). On the other hand, lower limb muscles impairments associated with cLBP, such as 35

knee extensor inhibition and reduction in hip extensor activation and fatigability, are often 36

reported in this population (24, 31, 52). More specific to runners, one recent study showed 37

that recreational runners with cLBP exhibited diminished knee extensor strength compared 38

with asymptomatic runners (4). The authors postulated that weakness of the knee muscles 39

may reduce capacity for shock attenuation, transmitting higher forces to the low back 40

during running. This suggests that strengthening lower limb muscles may contribute to 41

reducing loading and pain at the low back. Given that the treatment effects of conventional 42

lumbar extensor and lumbar stabilization exercise therapies remain inconclusive, it is worth 43

considering lower limb exercise therapy as an alternative approach to treat recreational 44

runners with cLBP for potentially better rehabilitation outcomes. 45

To fill the research gap of lacking evidence-based exercise therapy protocols to treat 46

runners with cLBP, this study aimed to evaluate the effectiveness of lower limb exercises, 47

4

compared with conventional lumbar extensor exercises and lumbar stabilization exercises, 48

in managing cLBP in recreational runners. We hypothesized that specific lower limb 49

exercises would be more effective in reducing running induced pain and improving self-50

rated running capability, lower limb strength, back muscles function, and running gait than 51

the conventional back exercises. 52

53

METHODS 54

A single-blind randomized trial was conducted to evaluate the effectiveness of lower limb 55

exercises, compared with conventional lumbar extensor exercises and lumbar stabilization 56

exercises, in managing cLBP in recreational runners. Ethical approvals were granted from 57

the Nanyang Technological University Institutional Review Board and the National 58

Healthcare Group Domain Specific Review Board. Ninety-one potential participants were 59

screened, of which 84 [male = 42] with mean age of 27.3 (5.5) years (TABLE 1, 60

Participants’ characteristics and exercise compliance) enrolled while waiting for 61

physiotherapy service in a hospital from November 8, 2013 to September 6, 2014. The 62

targeted sample size of 84 was calculated using G*Power 3.1 (medium effect size = 0.25, 63

power = 80%, α = .05, 3 groups for repeated measures). Written informed consents were 64

obtained and the rights of participants were protected. Using a block randomization table, 65

participants were evenly allocated into one of the three treatment groups (n = 28 per group 66

with even sex distribution): 1) lower limb (LL) exercises, 2) lumbar extensor (LE) exercises 67

and 3) lumbar stabilization (LS) exercises. The study flowchart of participants is shown as 68

FIGURE 1. 69

5

The inclusion criteria were: 1) 21 - 45 years old, 2) body mass index between 18 - 70

25 kg/m2, 3) had cLBP localized below the costal margin and above the inferior gluteal 71

folds for > 3 months and < 36 months, 4) running 2 - 5 times per week for ≥ 2 km per 72

session, and 5) started running six months prior to the study and have reached stable 73

training intensity for at least three months prior to the study. The exclusion criteria were: 1) 74

average pain intensity for the past one week < 2 or > 4 out of a 10-point Numeric Pain 75

Rating Scale (NPRS), 2) specific low back pain, e.g. spine fracture, disc herniation, nerve 76

root compression, 3) history of spine surgery, 4) current/history of lower limb conditions, 77

e.g., fracture, ankle sprain, patellar femoral pain syndrome, anterior tibial stress syndrome, 78

hip and knee arthritis and ligament laxity, 5) high fear-avoidance beliefs as determined by 79

the Fear-Avoidance Beliefs Questionnaire (FABQ)(14, 55) with physical activity score > 80

12 or work score > 19 (13), 6) work involving regular heavy lifting or hard physical work, 81

or 7) use of pain medication. 82

Outcome measures in current study consists of self-rated pain and running 83

capability, lower limb strength, back muscle function and running gait. Participants were 84

asked to rate their running induced pain using NPRS (0-10) according to the average rating 85

during the past one week. Similarly, self-rated running capability over the past one week 86

was measured using the Patient Specific Functional Scale (PSFS) (0 stands for unable to 87

perform running and 10 stands for able to perform running at the same level as before the 88

cLBP condition). The PSFS has been reported to be a more responsive functional outcome 89

measurement tool compared with other scales for chronic LBP rehabilitation in various 90

studies (17, 38, 42), especially in low activity limitation population (17), such as the 91

runners recruited in the current study. 92

6

Lower limb strength was assessed using an isokinetic dynamometer (Biodex system 93

4 Pro, Biodex Corp., Shirley NY, US) to measure the peak concentric torque at 60°/s for 94

knee extension, hip extension, and hip abduction. The test speed of 60°/s was reported to 95

demonstrate high test-retest reliability for knee extension (intra-class correlation 96

coefficients, ICC = .95) (49), hip abduction (ICC = .89) and hip extension (ICC = .90) (8) 97

isokinetic strength measurements. Both left and right limbs were tested, with orders 98

randomized. Details of the protocols are described elsewhere (4). The peak torque values 99

were normalized to body mass before being used for analysis. 100

Back muscle function was evaluated by lumbar stabilizing muscles activation and 101

lumbar extensor muscles fatigability, following the procedures previously used to assess 102

runners with cLBP (4). The transversus abdominis (TrA) and lumbar multifidus (LM) 103

activations, reflected by muscle percent thickness changes between resting and sub-104

maximal contraction (25), were measured using a rehabilitative ultrasound image (RUSI) 105

device (LOGIQ P5, GEHC, Milwaukee, WI, US) by a RUSI certified physiotherapist. The 106

test-retest reliability of this physiotherapist was excellent for both TrA ( ICC [95% CI] = 107

.96 [.89 - .98]; minimal detectable changes, MDC [95% CI] = 16.55%) and LM (ICC = .97 108

[.92 - .99], MDC [95% CI] = 6.54%) measurements. To reflect lumbar extensor muscle 109

fatigability, surface electromyography (EMG) signals of the bilateral iliocostalis and 110

longissimus during a 2-minute Sorensen test were also recorded at 1,000 Hz (Bagnoli™ 111

Desktop EMG system, Delsys® Boston, MA, US). Raw EMG data were band-pass filtered 112

at 20-450 Hz, and then analyzed in the frequency domain using a build-in software 113

(EMGworks® Software, Delsys® Boston, MA, US). The medium frequency was 114

determined from the power density spectrum obtained using the fast Fourier transform 115

7

technique with a Hamming windowing of 0.1 second. Finally, the medium frequency slope 116

(MFS) was calculated as the slope of the medium frequency plotted over time for each 117

muscle. 118

Spatio-temporal running gait parameters were measured using the OptoGait system 119

comprising two parallel bars (100 cm × 8 cm) mounted on each side of the treadmill 120

(Microgate S.r.I, Italy) which has been shown valid and reliable for gait analysis, with 121

accuracy within 1 cm (29, 30). Step length was calculated as the distance between the tip 122

(of the toe) of two successive foot contacts. The minimal contact time and flight time were 123

both set at 10 ms. Each participant was instructed to run at his/her usual comfortable speed 124

for 10 minutes. Running gait data during the 8-9th minute were collected at 1000 Hz as per 125

manufacturer’s specification. Subsequently, four gait parameters were extracted for 126

analysis: self-selected speed, step length, flight time, and contact time. 127

Participants were assessed on all outcome measures at pre-, mid- and end-128

intervention by a dedicated therapist who was blinded to treatment groups in the 129

physiotherapy clinic. For self-rated pain and running capability, as well as running gait, 130

additional follow-ups were done at three and six months post-intervention. Participants 131

were blinded from any previous ratings and results. 132

Participants were requested to attend supervised exercise sessions (LL, LE, or LS) 133

with their physiotherapists twice per week that were spread at least two days apart for eight 134

weeks. They were also asked to perform home exercises on other days of the week, guided 135

by an instruction sheet. For each supervised session, all participants performed a 136

standardized warm-up comprising general stretching exercises and stationary bicycling for 137

15 minutes, before their 30-minute specific exercise session with the therapist. 138

8

For the lower limb exercises group (LL), participants performed resistance exercises 139

targeting the knee and hip muscles for 8 weeks that has been shown to be effective in 140

increasing muscle strength of the targeted muscles (1). During the supervised exercise 141

sessions conducted in the physiotherapy department, a hip resistance training device was 142

used to strengthen the hip extensors (FIGURE 2a) and abductors (FIGURE 2b), and a leg 143

press machine was used to train the hip and knee extensors (FIGURE 2c). Taking into 144

account of participants’ safety and minimizing the risk of over-exercising, for each 145

exercise, participant performed 3 sets of 10 repetitions at an intensity of 10 repetition 146

maximum (RM) with two minutes of rest in between each set. The training intensity and 147

volume adopted here was recommended for muscle strengthening (27). The 10 RM was re-148

estimated at week 5 and the resistance was adjusted based on the new 10 RM for the 149

remaining four weeks of training. The training volume and frequency remained the same. 150

For the home exercises, single leg squat (FIGURE 2d) and wall-sit (FIGURE 2e) were 151

prescribed to participants instead. The single leg squat was reported to produce 82.3% of 152

maximal voluntary isometric contraction (MVIC) for hip extensor and 71.0% for hip 153

abductor (2). The wall sit as a close kinetic chain knee exercise was reported to produce 46 154

to 80% of MVIC of knee extension (50). Participants were asked to perform 3 sets of 10 155

repetitions of home exercises on days when there was no supervised exercise session. From 156

week 5 onwards, participants were instructed to hold a 2.5-kg weight during single-leg 157

squat and to hold a 5-kg weight during wall-sit. 158

For the lumbar extensor exercises group (LE), participants were prescribed an 8-159

week progressive back extensors training program to achieve physiological changes in 160

muscle fatigability (7, 44). To take care of participants’ safety and to prevent excessive 161

9

physical and psychological stress deriving from the exercise program, a progressive 162

approach was used. For the first week of training, participants performed leg raise in a 4-163

point kneel position with the lumbar spine in a neutral position during the leg flexion and 164

extension (FIGURE 3a). In the second week, participants performed contralateral leg and 165

arm raises (FIGURE 3b). This arrangement enabled participants to reach approximately 166

40% of MVIC at beginning of the second week without increasing the risk of injury to their 167

low back muscles (43). Three sets of 10 repetitions per session were performed for all 168

exercises, which recommended for muscle endurance and fatigability improvement (1). 169

Isometric contraction was also added to the end of each repetition as it was reported as an 170

essential component to improve the lumbar extensor fatigability (7, 39). Participants were 171

instructed to hold the end position for five seconds and rest for two seconds before the next 172

repetition. Two minutes of rest was given in between sets. In order to reach the 173

recommended intensity of approximately 60% of MVIC of lumbar extensors for 174

improvement in fatigability (39), a 0.5-kg of ankle weight was added at week 3 and a 0.5-175

kg of wrist weight was added at week 4. Subsequently, an increment of 0.5 kg every week 176

for the ankle and 0.5 kg every three weeks for the wrist were suggested to the participants 177

(FIGURE 3c). Prone back extension (FIGURE 3d) was introduced to replace the 4-point 178

kneeling exercises in week 5 because the percentage of MVIC produced by this exercise is 179

above 65% (9). Home exercises were identical to those in the supervised session, except no 180

prone back extension from week 5 onwards (FIGURE 3d). 181

For the lumbar stabilization exercises group (LS), participants received a series of 182

TrA and LM muscle activation and motor control training as previously described by 183

Koumantakis et al. (26). There were three stages of training: Stages 1 and 2 were 184

10

approximately two weeks and stage 3 about four weeks in duration. Participants were 185

allowed to progress to the next stage without being restricted by the timeline as soon as 186

they were able to complete the current stage of exercises satisfactorily. In stage 1, 187

participants were instructed to conduct low-load activation of the lumbar stabilizing 188

muscles, TrA and LM, with no movement (isometrically) and in minimal loading positions 189

of sitting and standing (FIGURE 4a and b). The RUSI was used to provide visual feedback 190

for TrA and LM activation. Excessive effort causing incorrect muscle activation in the 191

global muscles or spinal movement at the initial stages was discouraged. Progressively, the 192

holding time for each contraction was increased to at least 60 seconds and the duration of 193

each exercise session was increased up to 10 minutes (45). In stage 2, integration of the 194

lumbar stabilizing muscle activity into light dynamic functional tasks was added to 195

participant’s exercise programs as shown in FIGURE 4c and d. The participants were 196

instructed to practice with the same holding time and exercise duration as the first stage. In 197

stage 3, heavier-load functional tasks were progressively introduced to participants as 198

shown in FIGURE 4e with the resistance from the theraband during shoulder external 199

rotation in 70°-90° abduction and in FIGURE 4f with resistance during shoulder abduction 200

to 90°. For this stage, participants were instructed to practice the exercises for 10 minutes 201

twice a day (41). All exercises were used for both supervised exercise sessions and home 202

exercise, except the sitting balance integration component (FIGURE 4c), in which the gym-203

ball was substituted with a chair with cushion during home exercise. 204

All participants were informed that they should not feel exacerbation of their back 205

pain during training, and that the body reaction to exercise should be limited to “aching” or 206

“soreness”. Otherwise, the exercise intensity should be reduced or the program should be 207

11

terminated immediately. Participants were encouraged to continue their regular running but 208

refrain from heavy gym weight training during the entire 8-week exercise training and 6-209

month follow-up periods. Exercise logs were provided to document their home exercise 210

sessions, running frequency, and running distance. After completing the 8-week 211

intervention, participants were asked to stop their home exercises. 212

Statistical analyses in current study consist of covariate screening and treatment 213

effect comparisons. The participants’ characteristics, exercise compliant rate and running 214

distance were compared among the three groups by one-way ANOVA (for parametric data) 215

or Kruskal-Wallis test (for non-parametric data). Initially, variables with p-value smaller 216

than .10 were planned to be treated as covariates for the comparison of the treatment effects 217

but none of them had p-value smaller than .10 (TABLE 1). 218

A generalized estimating equation (GEE) approach using SPSS 21.0 was used to 219

compare the treatment and interaction effects. Given GEE’s capability of handling 220

outcomes with missing data and various correlations between time points (32), we could 221

include all 84 participants’ data in the analysis. The dependent variables entered to GEE 222

were the NPRS score, PSFS score, peak isokinetic torques, TrA and LM percent thickness 223

change, MFS for iliocostalis and longissimus, and running gait parameters. Targeted main 224

effects (group, time, sex) and interactions (group × time, group × sex, group × body side) 225

were entered as the independent variables to form the GEE models. A backward 226

elimination approach (α = .05) was applied during the model formation. Except for group 227

and time, independent variables which did not significantly contribute to the model were 228

removed. The model was then re-run with those significant independent variables. 229

230

12

RESULTS 231

In total, 74 participants completed all measurement sessions by April 27, 2015 (FIGURE 232

1). In the LL group, one participant injured her back during 5th week of the intervention 233

period and her subsequent follow-up data was not included in the analysis. The back injury 234

was not related to the study. There were also one participant loss of contact after his pre-235

intervention measurement session and one participant that missed out her final 236

measurement (6 months follow-up) due to migration. In the LE group, one participant 237

injured his ankle in the 3rd week of the intervention period by a minor traffic accident, and 238

his subsequent follow-up data was not included in the statistical analysis. There were also 239

one participant loss of contact after his initial measurement session, one participant loss of 240

contact after his mid-intervention measurement session (end of week 4) and one participant 241

missed out his end-intervention measurement session (end of week 8) due to busy schedule. 242

In the LS group, one participant injured her back during the 7th week of the intervention 243

period and her subsequent follow-up data was not included in the analysis. Another 244

participant injured his ankle during the 8th week of the intervention period, and thereafter 245

he was loss of contact, thus only his pre-intervention and mid-intervention measurement 246

data were included in the analysis. Both injuries were not related to the study. There was 247

also one participant in this group that missed out his end-intervention measurement session 248

(end of week 8) due to his busy schedule. 249

Among the three exercise groups, there was no significant difference in the 250

participants’ characteristics, compliant rate, or running habit (TABLE 1). The means (SD) 251

for all outcome measures during the intervention and follow-up periods are presented in 252

13

TABLE 2. Since group × body side did not contribute to any GEE model, the averaged 253

readings from both sides are presented in TABLE 2. 254

GEE analyses of all outcome measures are presented as below. For the NPRS score 255

of average running induced pain during the past 1 week, there was a main effect of time (p 256

< .001). Participants in all three groups achieved an average rate of improvement of 0.746 257

points over each time point [B = 0.746, 95% confidence interval (CI): (-0.799, -0.693), p = 258

.001]. Mean NPRS score differed across the three groups (p = .009). The LL group had 259

0.273 points lower mean NPRS score than the LE group [B = -0.273, 95% CI: (0.041, 260

0.505), p = .021], and 0.329 points lower than the LS group [B = -0.329, 95% CI: (0.088, 261

0.570), p = .008]. 262

For PSFS score, the changes in score significantly differed across the three groups 263

over time (group × time interaction, p < .001). LL group achieved an average rate of 264

improvement of 0.949 points over each time point [B = -0.949, 95% CI: (0.877, 1.021), p < 265

.001], which was 0.198 [B = -0.198, 95% CI: (-0.316, -0.080), p = .001] and 0.263 [B = -266

0.263, 95% CI: (-.406, -0.120), p < .001] more than the LE and LS groups, respectively. 267

For lower limb isokinetic strength, there was a significant group × time interaction 268

(p = .001) in peak knee extension torque. LL group improved on average 0.260 Nm/kg 269

over each time point [B = 0.260, 95% CI: (0.193, 0.326), p < .001], which was 0.220 270

Nm/kg [B = -0.220, 95% CI: (-0.307, -0.133), p < .001] and 0.206 Nm/kg [B = -0.206, 271

95% CI: (-0.306, -0.105), p < .001] higher than the LE and LS groups, respectively. Peak 272

knee extension torque differed between sexes (p < .001), with male participants presenting 273

0.276 Nm/kg higher compared to the female participants [B = 0.276, 95% CI: (0.115, 274

0.437), p = .001]. Peak hip extension torque increased over time [0.078 Nm/kg per time 275

14

point, 95% CI: (0.042, 0.113), p < .001] but did not differ across the three groups (p = 276

.154). Similarly, peak hip abduction torque increased over time [0.106 Nm/kg per time 277

point, 95% CI: (0.075, 0.137), p < .001], with no between-group difference (p = .363). 278

For lumbar stabilizing muscle activation, TrA percent thickness changes increased 279

on average 11.4% over each time point [B = 11.4, 95% CI: (8.30, 14.40), p < .001] with no 280

differences among the three groups (p = .061). There was a main effect of body side (p = 281

.001), with the dominant side exhibited 8.4% greater thickness change than the non-282

dominant side [B = 8.4, 95% CI: (3.50, 13.40), p = .001]. LM percent thickness changes 283

improved on average 9.2% over each time point [B = 9.2, 95% CI: (2.20, 16.20), p < .001] 284

but did not differ among the three groups (p = .188). 285

For lumbar extensor muscles fatigability measured using longissimus MFS, all three 286

groups slightly improved over each time point by 0.023 [B = 0.023, 95% CI: (0.007, 287

0.040), p = .005]. Means of MFS were different between male and female participants (p < 288

.001) and the LE group presented lower MFS than the LL group [B = -0.058, 95% CI: (-289

0.101, -0.015), p = .008]. For iliocostalis, the MFS differed across the three groups over 290

time (p = 0.033). While there was no change in MFS over time in LL [B = 0.006, 95% CI: 291

(0.008, 0.021), p = .398] and LS groups [B = 0.010, 95% CI: (-0.027, 0.047), p = .609], LE 292

group improved their MFS by 0.055 more over each time point [B = 0.055, 95% CI: (0.014, 293

0.097), p = .009]. 294

For running gait, self-selected running speed did not differ across the three groups 295

(p = .444) or change over time (p = .185). There was a main effect of sex (p < .001), with 296

male participants running 2.366 km/h faster than their female counterparts [B = 2.366, 95% 297

CI: (1.875, 2.856), p < .001]. Changes in running step length differed across the three 298

15

groups over time (p = .046). Participants in the LL group achieved an average increase of 299

2.464 cm in step length over each time point [B = 2.464, 95% CI: (0.953, 3.975), p = .001], 300

which was similar to the LE group [B = -1.690, 95% CI: (-3.639, 0.260), p = .089] but 301

greater by 2.213 cm per time point compared to the LS group [B = 2.213, 95% CI: (-3.959, 302

-0.468), p < .001]. Step length differed between sexes (p < .001), with longer step length in 303

male participants [B = 26.12, 95% CI: (21.384, 31.839), p < .001]. Flight time remained 304

stable with no changes over time (p = .208) and no difference across the three groups (p = 305

.931). Similarly, contact time also did not change over time (p = .356) or differ among the 306

three groups (p = .371). 307

308

DISCUSSION 309

This single-blinded randomized trial was conducted to evaluate the effectiveness of the 310

lower limb exercises, as compared with conventional back exercises, in managing non-311

specific cLBP in the recreational runner population. The study hypothesis that lower limb 312

exercises would be more effective in improving rehabilitation outcomes was partially 313

supported by our key findings: 1) greater improvement in self-rated running capability and 314

knee extension strength in LL group than LE and LS groups; 2) greater increase in running 315

step length in LL and LE groups than LS group; and 3) similar reduction in running 316

induced pain, and improvement in back muscle function across all three exercises groups. 317

Pain reduction is a key rehabilitation outcome in the treatment and management of 318

cLBP. This study showed that running induced pain improved over time for all participants 319

regardless of the exercise groups. At 6-month post-intervention, the total reduction in 320

NPRS score was 2.984 points (0.746 × 4 time points). This improvement exceeded the 321

16

MDC (95% CI) of 2.0 (6), and hence can be considered clinically significant. While there 322

are no studies directly comparing lower limb exercise to conventional back exercises on 323

pain reduction in runners with cLBP, others have shown that general exercise (which 324

included lower limb components) reduced back pain to a similar extent as specific lumbar 325

extensor or lumbar stabilization exercises (11, 33, 37). Thus, the reduction in running 326

induced pain observed among runners with cLBP in the present study is likely due to the 327

general effect of exercise rather than a specific type of exercise. 328

Using the PSFS to assess self-rated running capability, participants in the LL group 329

improved 3.796 (0.949 × 4 time points) at 6-month follow-up, compared to 3.004 for LE 330

and 2.744 points for LS groups, respectively. While all three groups responded positively to 331

the exercise treatments, it is important to note that only the LL group had achieved 332

clinically significant improvement by exceeding the MDC (95% CI) of 3.521 points for 333

single activity PSFS [calculated from the MDC = 3.0 (90% CI)] (51). This demonstrates 334

that lower limb exercise therapy is more effective than conventional back exercises in 335

improving self-rated running capability and therefore is a promising approach to treat cLBP 336

among runners. Previous studies on older, less active population revealed mixed results 337

when comparing the treatment effect of general exercise and lumbar stabilization exercise 338

on self-rated general functional outcome measured using Oswestry Low Back Pain 339

Disability Index or Roland Morris Disability Questionnaire (11, 33, 37). To our best 340

knowledge, the present study is the first to use PSFS-running as a specific functional 341

outcome to evaluate the effectiveness of exercise therapy in managing cLBP. Among the 342

various survey instruments commonly used to evaluate back pain, PSFS was reported to be 343

more responsive (effect size = 1.7) and specific for population with low physical activity 344

17

limitation (17) and hence this tool was chosen for the runners recruited in the current study. 345

Using a running specific functional outcome measurement, it is convincing to note the 346

superior treatment effect on the improvement in self-rated running capability achieved by 347

LL group than the other groups. 348

Regarding the lower limb isokinetic strength, we initially hypothesized greater 349

improvements in all hip and knee muscle strengths in the LL group compared with LE and 350

LS groups. This hypothesis was partially supported by our findings that peak knee 351

extension torque increased more in the LL group than the two conventional approaches but 352

similar improvements in hip extension and hip abduction torque were observed. By the end 353

of the 8-week intervention, peak knee extension torque increased by 31.82 Nm in total 354

[0.260 Nm/kg × 61.2 kg (mean body weight, TABLE 1) × 2 time points], compared with 355

4.8 Nm for LE and 6.5 Nm for LS groups, respectively. The improvement in the LL group 356

is of clinical importance as it has far exceeded the MDC (95% CI) of 17.88 Nm (49). 357

Comparing to the knee, the overall improvements in hip muscle strength were too small to 358

be practically meaningful (hip extension: 9.55 Nm [MDC (95% CI) = 28.82 Nm], hip 359

abduction: 12.97 Nm [MDC (95% CI) = 34.00 Nm]) (8). 360

It is interesting to note that among all muscle functions tested, lower limb exercises 361

only induced greater improvement in knee extensor strength compared with conventional 362

back exercises. Other functions including hip muscle strength, lumbar stabilizing muscle 363

activation, and lumbar extensor muscle fatigability were similarly impacted regardless of 364

type of exercises prescribed. This suggests that the higher self-rated running capability 365

observed in the LL group is likely related to greater gain in knee strength, supporting a 366

previous speculation that weak knee extensors may compromise one’s ability to absorb 367

18

impact shock during running and hence transmitting higher forces to the low back (4). 368

Thus, improving knee extensor strength may be the step needed to break the vicious cycle 369

of knee and back muscle dysfunction previously reported in LBP population (19, 46). 370

In current study, similar improvements in lumbar stabilizing muscles activation are 371

seen across all three groups for both muscles. By the end of the 8-week intervention, TrA 372

activation improved by 22.8% which was approaching the MDC (95% CI) of 25.4% (25). 373

Similarly, LM activation improved by 18.4% at end of 8 weeks and this change overcame 374

the MDC (95% CI) of 11.0% (25). One previous study reported non-exercise specific 375

improvement in TrA activation when comparing lumbar extensor and lumbar stabilization 376

exercises in the general LBP population (54). Lower limb weight bearing exercises were 377

also observed to induce similar TrA activation as back muscle exercises (21). For the LM, 378

similar EMG activations were reported for different types of exercises that closely 379

resembled those adopted in the current study (18, 40, 53). Collectively, our results are in 380

line with the literature that clinically meaningful improvements in lumbar stabilizing 381

muscles activation can be achieved via exercise training in general. However, improvement 382

in longissimus fatigability, despite showing statistical significance, is unlikely clinically 383

meaningful since the changes in MFS by 0.046 (0.023 × 2 time points) after 8-week 384

training were too small to overcome the MDC (95% CI) ranging from 0.11 to 0.17 (10). 385

The lack of improvement in longissimus fatigability parallel with previous studies 386

examining the training effect of lumbar extensor (26, 39), stabilization (26), and general 387

lower limb exercises (36). Interestingly, improvement in iliocostalis fatigability in the LE 388

group is about clinically meaningful, with MFS changes of 0.11 (0.055 × 2 points) after 8 389

weeks. This finding contradicted a previous study reporting no change in muscle 390

19

fatigability after a 12-week lumbar extensor isoinertial exercise intervention (36). 391

Differences in exercise protocol (isometric vs isoinertial) and EMG electrodes placement 392

(parallel vs 45° to the spine) (36) may explain the lack of agreement between the previous 393

and present study. In summary, the results from the present study suggest that lower limb 394

exercises are equally effective as back exercises in improving lumbar stabilizing muscle 395

activation in runners with cLBP. On the other hand, lumbar extensor muscle fatigability 396

was responsive to lumbar extensor exercises but not lower limb or lumbar stabilization 397

exercises. 398

Running step length as the only running gait parameter significantly changed by 399

exercise training in current study increased by 9.856 cm (2.464 cm × 4 time points) by the 400

end of the 6-month follow-up in the LL group, which was similar to LE group but 401

significantly more than LS group. Since the running speeds on treadmill are rather stable 402

over time (TABLE 2), increased step length would have resulted in reduced their step 403

frequency given that speed is a product of step length and step frequency. Taking fewer 404

steps to complete the same distance may have reduced the number of impacts on the spine 405

during the ground contact (5), contributing to reduced pain and improved running 406

capability reported by the participants. While the OptoGait system has been shown valid 407

and reliable for measuring spatiotemporal parameters during walking (29, 30), there is no 408

established MDC to determine the clinical relevance of any observed changes during 409

treadmill running. Nevertheless, it is clear that lower limb exercises lead to the greatest 410

increase in running step length among our participants. 411

There are a few limitations to the current study. First, the low attendance of 412

supervised exercise session [mean 5.6 (4-7) visits out of 16] was less than the minimal 413

20

frequency of once a week for successful treatment in cLBP patients (3). Home exercise 414

compliance rate was much better [mean 29.3 (13-52) sessions], suggesting that any 415

improvements observed were most likely attributed to home exercises rather than 416

supervised training. In the literature, home exercise compliance rather than formal 417

physiotherapy session attendance was found correlated to the reductions in pain and self-418

reported disability in cLBP patients (34). Future studies can investigate whether home 419

exercise alone is sufficient to successfully treat cLBP conditions. Second, only 420

spatiotemporal gait variables during a treadmill test were measured. Since altered trunk 421

posture during running in cLBP population has been reported, (48), additional kinematics 422

and kinetic data will be useful for a more comprehensive biomechanical evaluation of 423

running gait. Lastly, the current findings should be applied with caution to older, less active 424

individuals since our participants were younger [age = 27.3 (5.5) year)], recreational 425

runners. 426

427

CONCLUSION 428

Lower limb exercise therapy has shown to be a promising approach to the clinical 429

management of non-specific cLBP in recreational runners. Compared with conventional 430

back exercises, lower limb exercise therapy was more effective in improving key 431

rehabilitation outcomes including self-rated running capability, knee extension strength, 432

and running step length. All exercise therapies were equally effective in reducing running 433

induced pain and improving back muscle function. 434

435

Acknowledgments 436

21

The authors would like to thank the National Institute of Education Academic Research 437

Fund, Singapore for providing the financial support for this project and the management 438

of Allied Health Division, Jurong Health Services, Singapore, for providing experiment 439

venue and clinical hours to complete the project. 440

We would like to thank Mr Johnny Wong from Clinical Research Unit, Ng Teng Fong 441

General Hospital, Jurong Health Services, Singapore, for his valued advice on the statistical 442

analysis of the current study. 443

444

Conflict of Interest 445

This study was funded by the National Institute of Education Academic Research Fund. 446

The funding source did not play a role in the investigation. We affirm that we have no 447

financial affiliation (including research funding) or involvement with any commercial 448

organization that has a direct financial interest in any matter included in this manuscript. 449

The results of the present study do not constitute endorsement by ACSM. 450

The results of the study are presented clearly, honestly, and without fabrication, 451

falsification, or inappropriate data manipulation. 452

453

22

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FIGURE 1. The study flowchart of participants.

30

FIGURE 2. Lower limb exercises during supervised sessions and home exercises.

Supervised sessions included resistance training exercises: hip abductors (a), hip extensors (b), leg press

(c). Home exercises comprised single leg squat (d) and wall-sit exercise (e).

31

FIGURE 3. Lumbar extensors exercises

For the first week of training, participants performed leg raise in a 4-point kneel position with the

lumbar spine in a neutral position (a). In the second week, participants performed contralateral leg and

arm raises (b). A 0.5-kg of ankle weight was added at week 3 and a 0.5-kg of wrist weight was added at

week 4. Subsequently, an increment of 0.5 kg every week for the ankle and 0.5 kg every three weeks for

the wrist were suggested to participants (c). Prone back extension (d) was introduced to replace the 4-

point kneeling exercises in week 5 only during the supervised exercise sessions but not for the home

exercises (i.e. participants continued with their contralateral leg and arm raises with ankle and wrist

weights at home).

32

FIGURE 4. Lumber stabilization exercises

In stage 1, participants were instructed to conduct low-load activation of transversus abdominis and

lumbar multifidus, isometrically in minimal loading positions of sitting and standing (a-b). In stage 2,

integration of the lumbar stabilizing muscle activity into light dynamic functional tasks was added to

participant’s exercise programs (c-d). In stage 3, heavier-load functional tasks were progressively

introduced to participants with the resistance from the theraband during shoulder external rotation in 90°

abduction (e) and with the resistance during shoulder abduction in a single leg stance position (f).

TABLE 1. Participants’ characteristics and exercise compliance

All Participants (n = 84)

LL group (n = 28)

LE group (n = 28)

LS group (n = 28)

p-value

*Age (years) 27.3 (5.5) 28.9 (5.3) 26.1 (4.1) 26.9 (6.4) .154 *Body mass (kg) 61.2 (11.8) 61.7 (12.6) 61.7 (10.8) 60.3 (12.1) .877 *Body mass index (kg/m2) 21.8 (2.4) 21.7 (2.4) 21.8 (2.4) 21.9 (2.4) .948 *Usual running frequency per week 2.6 (0.8) 2.5 (0.8) 2.6 (0.7) 2.5 (0.8) .772 *Usual running distance (km) per time 4.1 (2.0) 4.2 (1.8) 3.8 (2.0) 4.3 (2.3) .657 *Pain duration (weeks) 65.4 (31.6) 62.0 (35.0) 65.4 (33.1) 68.9 (27.0) .722 *†Total supervised sessions attended during 8 weeks 5.6 (2.1) 5.9 (2.3) 5.4 (2.3) 5.4 (1.9) .629 *†Total home exercise sessions during 8 weeks 29.3 (11.3) 29.1 (10.0) 28.9 (12.6) 29.9 (11.9) .947 *†Total running distance (km) during 8 weeks 40.1 (21.1) 37.5 (21.6) 36.4(19.2) 46.2 (21.8) .181 ‡Back pain region

Left 6 (7.1%) 6 (7.1%) 3 (3.6%)

.770 right 6 (7.1%) 5 (6.0%) 4 (4.8%) bilateral 9 (10.7%) 11 (13.1%) 15 (17.9) central 7 (8.3%) 6 (7.1%) 6 (7.1)

‡Pain onset gradual 26 (31.0%) 21 (25.0%) 25 (29.8%)

.130 sudden 2 (2.4%) 7 (8.3%) 3 (3.6%)

‡History of previous LBP episode/s yes 10 (11.9%) 14 (16.7%) 12 (14.3%)

.588 no 18 (21.4%) 14 (16.7%) 16 (19.0%) *Values are mean (SD). †based on 75 participants (5 loss of contact and 4 injured participants were excluded from the analysis; LL group, n = 26); LE group, n = 24 and LS group, n = 25. ‡ Values are counts (% of the total counts). LL = lower limb exercises, LE = lumbar extensor exercises, LS = lumbar stabilization exercises, LBP = low back pain.

TABLE 2. Self-rated outcomes, running gait measurements, and muscle functional characteristics during intervention and follow-ups

Pre Mid End 3 months 6 months NPRS-average running induced pain during the past 1 week

LL 3.48 (1.00) 0.67 (0.72) 0.32 (0.48) 0.20 (0.41) 0.12 (0.33) LE 3.44 (0.87) 1.36 (1.00) 0.76 (0.78) 0.60 (0.76) 0.24 (0.52) LS 3.62 (1.13) 1.15 (0.97) 0.65 (0.56) 0.62 (0.75) 0.50 (0.58)

PSFS-running LL 6.52 (0.90) 7.92 (0.74) 9.23 (0.65) 9.73 (0.53) 9.92 (0.27) LE 6.71 (0.92) 7.69 (0.74) 8.65 (0.85) 9.35 (0.80) 9.65 (0.49) LS 6.62 (0.90) 8.00 (0.85) 8.81 (0.80) 9.27 (0.67) 9.35 (0.75)

Self-selected running speed (km/h)

LL 8.65 (1.66) 8.64 (1.52) 8.69 (1.38) 8.87 (1.53) 8.84 (1.84) LE 8.83 (2.28) 8.63 (1.57) 8.52 (2.00) 9.03 (2.28) 8.53 (1.80) LS 8.49 (1.87) 8.84 (1.76) 8.64 (1.81) 8.28 (1.49) 9.08 (1.74)

Step length (cm) LL 83.18 (24.63) 85.90 (18.09) 87.40 (19.09) 88.68 (17.49) 92.64 (19.06) LE 85.25 (20.28) 85.67 (17.15) 85.52 (19.21) 89.65 (22.31) 88.10 (19.50) LS 85.82 (20.92) 91.36 (19.71) 86.86 (20.20) 84.82 (16.84) 87.12 (18.63) Flight time (s) LL 0.107 (0.039) 0.106 (0.040) 0.106 (0.037) 0.104 (0.034) 0.108 (0.032) LE 0.104 (0.043) 0.108 (0.035) 0.111 (0.039) 0.112 (0.037) 0.117 (0.038) LS 0.107 (0.035) 0.105 (0.036) 0.105 (0.032) 0.107 (0.029) 0.109 (0.031) Contact time (s) LL 0.235 (0.051) 0.250 (0.048) 0.248 (0.044) 0.249 (0.045) 0.251 (0.032) LE 0.258 (0.046) 0.255 (0.032) 0.251 (0.039) 0.243 (0.041) 0.237 (0.042) LS 0.259 (0.038) 0.257 (0.033) 0.258 (0.036) 0.256 (0.037) 0.250 (0.031) TrA percent thickness changes (%)

LL 61.33 (29.67) 66.97 (31.74) 80.97 (31.31) -- -- LE 49.55 (26.42) 59.76 (22.06) 65.34 (22.72) -- --

LS 40.19 (20.13) 56.25 (31.22) 72.10 (35.03) -- -- LM percent thickness changes (%)

LL 23.44 (8.77) 29.48 (9.54) 55.04 (26.04) -- -- LE 24.31 (11.60) 28.22 (9.91) 34.10 (12.37) -- --

LS 19.05 (11.05) 25.95 (12.73) 31.61 (15.61) -- -- Knee extension (Nm/kg)

LL 1.97 (0.45) 2.37 (0.39) 2.47 (0.38) -- -- LE 2.23 (0.39) 2.29 (0.32) 2.30 (0.40) -- --

LS 2.16 (0.48) 2.21 (0.45) 2.25 (0.44) -- -- Hip extension (Nm/kg) LL 1.22 (0.42) 1.37 (0.40) 1.42 (0.40) -- -- LE 1.23 (0.41) 1.33 (0.53) 1.40 (0.53) -- -- LS 1.12 (0.34) 1.23 (0.28) 1.21 (0.32) -- -- Hip abduction (Nm/kg) LL 1.45 (0.28) 1.58 (0.29) 1.65 (0.29) -- -- LE 1.44 (0.35) 1.48 (0.23) 1.58 (0.35) -- -- LS 1.31 (0.25) 1.46 (0.20) 1.58 (0.27) -- -- Longissimus MFS LL -0.19 (0.08) -0.21 (0.08) -0.16 (0.10) -- -- LE -0.28 (0.18) -0.17 (0.12) -0.20 (0.13) -- -- LS -0.18 (0.11) -0.20 (0.10) -0.18 (0.09) -- -- Iliocostalis MFS LL -0.17 (0.08) -0.16 (0.11) -0.16 (0.09) -- -- LE -0.27 (0.18) -0.16 (0.13) -0.16 (0.12) -- -- LS -0.14 (0.18) -0.17 (0.11) -0.13 (0.10) -- --

Values are mean (SD). Data of running gait and muscle characteristics were averaged value of dominant and non-dominant sides. LL = lower limb exercise group, LE = lumbar extensor exercise group, LS = lumbar stabilization exercise group, PSFS = patient specific functional scale, NPRS = numeric pain rating scale, TrA = transversus abdominis, LM = lumbar multifidus, MFS = mean frequency slope. The Pre (pre-intervention) data were based on LL group, n = 28, LE group, n = 28 and LS group, n = 28. The Mid (mid-intervention) data were based on LL group, n = 27, LE group, n = 26 and LS group, n = 28. The End (end-intervention) data were based on LL group, n = 26, LE group, n = 24 and LS group, n = 25. The 3 months data were based on LL group, n = 26, LE group, n = 24 and LS group, n = 25. The 6 months data were based on LL group, n = 25, LE group, n = 24 and LS group, n = 25.